Conventionally, monitoring cameras are used for the purpose of security, recording, etc. It is required for an optical system for use with such a monitoring camera to be compact and able to be formed inexpensively, have high aperture ratio to allow identifying the subject even under a low illuminance imaging condition, have high zoom ratio to be able to accommodate from the wide-angle end, at which a wide-range image can be taken, to the telephoto end, at which a narrower range can be enlarged, and have high optical performance.
For an optical system for use with a day- and night-vision monitoring camera, which is often placed at an unmanned facility and performs visible light imaging during day and near-infrared light imaging during night, it is desired to maintain high optical performance with well corrected chromatic aberration over a wide wavelength range from the visible range to the near-infrared range.
As a variable magnification optical system with well corrected chromatic aberration from the visible range to the near-infrared range, one disclosed in U.S. Pat. No. 7,907,351 (hereinafter, Patent Document 1), for example, is known. The optical system disclosed in Patent Document 1 includes, in order from the object side, a negative first lens group and a positive second lens group, where magnification change is achieved by moving the second lens group along the optical axis, and correction of the image plane position along with the magnification change is achieved by moving the first lens group. Further, examples of a variable magnification optical system having a two-group configuration, where the magnification change is achieved by changing the interval between two lens groups, are disclosed, for example, in U.S. Pat. Nos. 7,280,284 and 6,888,682 (hereinafter, Patent Documents 2 and 3).